1. Field of the Invention
The present invention relates to an apparatus for detecting a line accident such as a short circuit of a line and a short with a ground of line, utilizing an optical sensor for detecting or measuring a zero-phase current or a zero-phase voltage in a transmission line or a distribution line.
2. Related Art Statement
Recently, it is necessary to keep an accurate and prompt power supply to satisfy a variety of demands for the power supply. Correspondingly, the supply of electric power and the management institution thereof are complicated and varied, and thus promote development of a practical automatic system. Moreover, the collection of information for this power supply, e.g. the detection and measurement of voltage and current in a line, must be performed in an accurate and prompt manner at a plurality of points.
Heretofore, apparatus for detecting a short circuit in a line such as a polyphase AC line, particularly a three-phase AC line, are known such as the apparatus for detecting and measuring the zero-phase current or the zero-phase voltage shown in FIGS. 4 and 5.
As shown in FIG. 4, in the apparatus for detecting and measuring the zero-phase current, lead wires 53 of respective current transformers 52 arranged in an electric wire 51 are connected in series, and a resistor 54 is connected to both ends of the series lead wires 53. Then, a voltage in response to a current variation in respective electric wires 51 is generated at output terminals 55, 55.
In the apparatus having the above mentioned construction, the electric signal normally supplied from the output terminals 55, 55 is the zero-phase current in which a sum of current phases is zero. However, when a line accident occurs, the electric signal supplied from the output terminals 55, 55 does not indicate a zero value, and the electric signal in response to a scale of line accident is generated.
Contrary to this, in the apparatus for detecting and measuring the zero-phase voltage shown in FIG. 5, a resistor 54 is connected to lead wires 53 of a three-phase voltage transformer or three single-phase voltage transformers 56 whose first terminals are connected to electric wires 51 in Y connection, and a voltage in response to a current variation in respective electric wires 51 is generated at output terminals 55, 55.
In the apparatus having the above mentioned construction, the electric signal normally supplied from the output terminals 55, 55 is the zero-phase voltage in which a sum of voltage phases is zero. However, when a line accident occurs, the electric signal output from the output terminals 55, 55 does not indicate a zero value, and the electric signal in response to a scale of line accident is generated.
In this manner, the line accident is detected by detecting and measuring the zero-phase current or the zero-phase voltage or further both zero-phase current and voltage.
However, in the known apparatuses mentioned above, since electromagnetic induction is induced in the lead wires 53 of the current transformer 52 or the voltage transformer 56, there occurs the drawback that the S/N ratio is decreased and thus the measurement accuracy becomes bad. Especially, since it is necessary to improve the reliability of the transmission line and the distribution line to detect accidents such as instant short with ground of line, or intermittent short with ground of line, which cannot be detected until now, the highly precise detection of line voltage or line current must be realized. Moreover, there occur the drawbacks that the short circuiting of the line happens to be due to the superannuation of the lead wire 53 and the short with ground of line happen to be due to the breakage of insulation of the current transformer 52 or the voltage transformer 56.